Breast cancer accounts for 29 percent of all cancers in women, and approximately 43,500 deaths in the United States in 1998. Current breast cancer treatment calls for the removal of either all or a portion of the breast including axillary lymph nodes, followed by radiation or chemotherapy. The broad long-term objective of the proposed project is to optimize a minimally- invasive laser thermal therapy approach to breast cancer treatment that is currently being investigated as an alternative to conventional surgical removal. The laser procedure will reduce the treatment cost and maintain the original cosmetic appearance of the patient, avoiding physical, emotional, and psychological distress due to disfigurement. The specific aims of the proposed research are to obtain the optimum laser treatment parameters and fiber design for laser thermal treatment of breast cancer. The relationship between the optical properties of the tissue and the temperature distribution will be determined by thermal experiments in a tissue phantom with variable optical properties, as well as in ex vivo porcine fatty tissue, human cadaver breast tissue, and in vivo porcine fatty tissue. The laser energy will be delivered through optical fibers with cylindrical diffusing tips. Based on these experiments, a thermo-optical model will be developed to calculate optimal laser treatment parameters to safely and efficiently produce the desired temperature distribution.